Wrong causal direction:

I’ve noticed that people with psychological disorders tend to use recreational drugs. They must be taking drugs to relieve the trauma of their disorders.

Notes

Without evidence to indicate causation and the direction of that causation, the coincidence of two phenomena is at best correlation.

Case Study One

The increase of handguns has often been cited as the cause of increasing crime. Before this claim can be rigorously substantiated, the degree that crime has had on the increase in handguns purchased for increased personal security in the area of increased crime will have to be teased out.

Case Study Two

In the middle ages, Europeans noticed that people who got sick had fewer fleas on their bodies, leading them to conclude that fleas prevent illness. The correct explanation was that an illness was usually accompanied by a fever in its late stages, which then forced the heat-sensitive fleas to look for another host.

Case Study Three

A correlation between being married and good health may indicate 1) that marriage causes good health, 2) that good health causes marriage, 3) that there is a recursive relationship between the two, 4) that there is another factor that is the cause of both marriage and good health, or 5) be simply a random correlation without any causal connection.